This invention relates to microwave distribution systems, and, more particularly, to a stripline structure used in a microwave system.
Microwave energy is employed to transmit communications signals because of its high frequency and the consequent ability to convey a large amount of information, and because it may be amplified to high power levels. For example, extremely high frequency (EHF) energy in the 15-40 GHz (gigahertz) range is used in many communications applications. The communications signals conveyed through communications satellites are transmitted from an earth ground station through free space to the satellite in geosynchronous orbit. The signals are there amplified by an on-board amplifier and retransmitted through free space to another earth ground station.
When the microwave signals are being amplified and otherwise processed on board the satellite, they are conveyed in waveguides and/or on thin metallic substrates termed striplines. At some points in the distribution system, waveguides are too large in physical dimensions too heavy, and too complex to be practical. For example, microwave signals conveyed from and to the segmented antennas of the satellite must be combined when received and divided when transmitted. A waveguide system may be used for these purposes, but it is large, heavy, and complex. A stripline system is much smaller, lighter, cheaper, and less complex, but it exhibits a higher signal attenuation than the waveguide.
There is therefore a tradeoff between the two approaches. The stripline system would be more attractive in applications such as antenna systems if it could be built to be lighter and less costly than possible with presently available approaches. Accordingly, there is a need for a better approach to microwave stripline structures which are particularly suited for packing a large number of stripline conductors into a small space. The present invention fulfills this need, and further provides related advantages.
The present invention provides a stripline structure suitable for conducting microwave signals. The stripline structure is compact and extremely light in weight. It is constructed from available, space-qualified materials, and may be readily fabricated. Its radio frequency attenuation is acceptable, while maintaining the mechanical rigidity for use in spacecraft. The stripline structure may be sized to be suitable for use with a wide range of microwave frequencies, including the 15-40 Gigahertz extremely high frequency range that is desirable for communications satellites. The stripline structure is designed for efficient scale-up to a multichannel form that accommodates a large number of signals on individual stripline conductors. The stripline structure is thus particularly useful for combiner/divider applications such as those used to carry signals from and to microwave antennas.
In accordance with the invention, a microwave stripline structure includes a first-layer conductor structure comprising a nonmetallic central conductor substrate having a first side and a second side,band a first ground plane layer spaced apart from the first side of the central conductor substrate. The first ground plane layer includes a first ground plane substrate, preferably comprising a nonmetallic material, and a first metallic layer structure contacting at least one side of the first ground plane substrate. The stripline structure further includes a first foam layer disposed in contact with the first side of the central conductor substrate and with the first ground plane layer. The first foam layer optionally but preferably has a first channel therethrough with the first foam layer, the central conductor substrate, and the ground plane layer bounding the first channel. An elongated metallic central conductor is present on the first side of the central conductor substrate, within the first channel in the embodiments having the first channel.
The stripline structure further includes a second ground plane layer spaced apart from the second side of the central conductor substrate. The second ground plane layer includes a second ground plane substrate, preferably comprising a nonmetallic material, and a second metallic layer structure contacting at least one side of the second ground plane substrate. A second foam layer may be disposed in contact with the second side of the central conductor substrate and with the second ground plane layer. The second foam layer optionally but preferably has a second channel therethrough in registry with the first channel, with the second foam layer, the central conductor substrate, and the ground plane layer bounding the second channel. It is preferred that the central conductor substrate, the first ground plane layer, and the second ground plane layer are substantially planar and parallel to each other to a tolerance of within +/xe2x88x920.001 inch.
In a form particularly suitable for a multichannel, stacked arrangement, a microwave stripline structure includes a first-layer conductor structure comprising a substantially planar nonmetallic central conductor substrate having a first side and a second side. The central conductor substrate preferably comprises a composite material of fibers embedded in a cured resin. There is a substantially planar first ground plane layer spaced apart from the first side of the central conductor substrate. The first ground plane layer includes a first ground plane substrate, preferably comprising a nonmetallic material, and a first metallic layer structure on the first ground plane substrate. The first metallic layer structure includes a first metallic inner layer facing the central conductor substrate and a first metallic outer layer disposed remotely from the central conductor substrate. There is a substantially planar first foam layer in contact with the first side of the central conductor substrate and with the first ground plane layer. The first foam layer has a first channel therethrough with the first foam layer, the central conductor substrate, and the ground plane layer bounding the first channel. An elongated metallic central conductor is positioned on the first side of the central conductor substrate within the first channel. There is a substantially planar second ground plane layer spaced apart from the second side of the central conductor substrate. The second ground plane layer includes a second ground plane substrate, preferably comprising a nonmetallic ;material, and a second metallic layer structure on the second ground plane substrate. The second metallic layer structure includes a second metallic inner layer facing the central conductor substrate and a second metallic outer layer disposed remotely from the central conductor substrate. There is additionally a substantially planar second foam layer disposed in contact with the second side of the central conductor substrate and with the second ground plane layer. The second foam layer has a second channel therethrough, in registry with the first channel, with the second foam layer, the central conductor substrate, and the ground plane layer bounding the second channel. Optionally, a nonmetallic post may extend through the central conductor substrate, the first ground plane layer, the first foam layer, the second ground plane layer, and the second foam layer.
Stated alternatively, a microwave stripline structure includes a first-layer conductor structure having a first suspended stripline conductor comprising a planar nonmetallic central conductor substrate having a first side and a second side, an elongated metallic central conductor on a first side of the central conductor substrate, and two planar ground plane, layers. One ground plane layer is in facing-but-spaced apart relation to each side of the central conductor substrate. Each ground plane layer comprises a ground plane substrate, preferably made of a nonmetallic material, and a metallic layer structure contacting at least one side of the ground plane substrate. The stripline structure further includes two planar foam layers. Each foam layer contacts one side of the central conductor substrate and one of the ground plane layers. Each foam layer has a channel therethrough in registry with the channel of the other foam layer, with the elongated metallic central conductor lying within one of the channels. The respective foam layer, the central conductor substrate, and the respective ground plane layer bound each channel.
In any of these embodiments, the basic stripline structure may be readily expanded to a multichannel form. In one approach involving an in-plane expansion, the first-layer conductor structure has at least one additional stripline conductor, with each additional stripline conductor having a structure substantially identical to the first stripline conductor. In a second approach involving a parallel-plane expansion, a second-layer conductor structure is in facing relation to the first-layer conductor structure. The second-layer conductor structure has the same structure as the first-layer conductor structure.
Typically, the ground plane substrates in the various embodiments comprise a flexible absorber material having an electrical resistance of about that of free space (i.e., about 377 ohms). The ground plane substrates are each preferably a layer of semi-conductive, absorptive fibers. The central conductor substrate comprises a composite material of quartz fibers embedded in a cured cyanate ester resin. The foam layers comprise an electrically nonconductive, closed-cell foam such as polymethacrylimide foam.
A feature of the preferred form of the invention is that it contains no polytetrafluoroethylene (sometimes known as Teflon(trademark)) polymer. This material is difficult to bond and usually requires a housing to mechanically position it. Further, it has a tendency to cold flow in a space environment. The presently preferred approach uses no polytetrafluoroethylene.
The present approach provides a light weight, strong, readily manufactured stripline structure. The basic design may be expanded to a large number of applications using in-plane or parallel-plane arrangements. Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.